This invention relates to tube encased fiber optic pressure sensors, and more particularly to fluid ingression protection mechanisms for a tube-encased fiber grating pressure sensor.
Sensors for the measurement of various physical parameters such as pressure and temperature often rely on the transmission of strain from an elastic structure (e.g., a diaphragm, bellows, etc.) to a sensing element. In a fiber optic pressure sensor, the sensing element may be encased within a glass tube or housing comprised substantially of glass. One example of a fiber optic based sensor is that described in U.S. patent application Ser. No. 09/455,867, entitled xe2x80x9cBragg Grating Pressure Sensor,xe2x80x9d filed Dec. 6, 1999, which is incorporated herein by reference in its entirety.
The use of fiber optic based devices is widespread in the telecommunications industry, wherein the impervious nature of the glass provides adequate protection given the relatively mild working environments. A relatively recently known use of fiber optic pressure sensors is in an oil well to measure temperature and pressure at various locations along the length of the well bore. The sensors are typically deployed in metal housings in the wellbore and are attached on the outside of the casing. Such sensors may often be subjected to extremely harsh environments, such as temperatures up to 200 degrees C. and pressures up to 20 kpsi. These sensors are exceptionally sensitive and are capable of measuring various parameters, such as temperature and pressure, with extreme accuracy. However, the sensitivity and accuracy of fiber optic sensors creates problems when such sensors are used in a harsh environment. Known problems include poor signal to noise ratios, wavelength drift, wavelength shifts, optical losses, hysteresis and mechanical reliability issues. It is the realization of the these problems and the discovery of the causes that will advance the state of the art in fiber optic based well bore monitoring systems.
One such known problem is xe2x80x9ccreepxe2x80x9d of the sensor over time. It has been discovered that the attachment of the sensing element to the elastic structure can be a large source of error if the attachment is not highly stable. In the case of sensors that measure static or very slowly changing parameters, the long-term stability of the attachment to the structure is extremely important. A major source of such long-term sensor instability is creep, i.e., a change in strain on the sensing element even with no change in applied load on the elastic structure, which results in a DC shift or drift error in the sensor signal. Various techniques now exist for attaching the fiber to the structure to minimize creep, such as adhesives, bonds, epoxy, cements and/or solders.
In addition, the sensors are subject to fluids containing hydrocarbons, water, and gases that can have deleterious effects on the accuracy of the sensors. For instance, it has been discovered that the performance of wellbore deployed fiber optic sensors is adversely affected by exposure to hydrogen, which causes irreversible loss along the fiber""s length. Further, when the fiber optic sensors include Bragg gratings, exposure to hydrogen causes a shift in the index of the grating that severely lessens the accuracy of the sensor. Increased pressure and temperature of the hydrogen increases the rate at which the fiber optic cables and sensors degrade.
It has also been discovered that certain side-hole fiber optic pressure sensors and eccentric core optical fiber sensors experience deleterious effects, such as those described above, when exposed to water at high temperatures and pressures. The adverse effects are presumed to be caused by thin swollen surface layers that lay in close proximity to the sensitive fiber optic core. The observed shifts and changes are presumed to be due to the ingress of water molecules and the subsequent direct expansion of the silica that makes up the fiber itself. In one particular instance, the fibers had a core center-to-surface separation distance of only 10 xcexcm.
However, as discussed hereinbefore, many other problems and errors associated with fiber optic sensors for use in harsh environments still exist. There is a need to discover the sources of these problems and errors and to discover solutions thereto to advance the state of the art in fiber optic sensor use.
Objects of the present invention include a fiber optic pressure sensor with fluid blocking provisions for use in a harsh environment.
According to the present invention, a fluid blocking fiber optic pressure sensor comprises an optical fiber having at least one pressure reflective element embedded therein, wherein the pressure reflective element has a pressure reflection wavelength; a sensing element having the optical fiber and the reflective element encased therein, the sensing element being fused to at least a portion of the fiber and being strained due to a change in external pressure whereby the strain causes a change in the pressure reflection wavelength indicative of the change in pressure; and a fluid blocking coating disposed on the external surface of the sensing element.
According further to the present invention, the sensing element comprises a tube and the fluid blocking coating comprises at least one layer. The fluid blocking coating comprises a fluid blocking material of gold, chrome, silver, carbon, silicon nitride, or other similar material capable of preventing the diffusion of water molecules into to the sensing element. Alternatively, the coating comprises a first layer comprised of chrome disposed on the outside surface of the sensing element and a second layer comprised of gold disposed on the first layer. In one embodiment, the first layer has a thickness of about 250 xc3x85 and the second layer has a thickness of about 20,000 xc3x85.
The present invention also provides a fluid blocking fiber optic pressure sensor having a fiber grating encased in and fused to at least a portion of a sensing element, such as a capillary tube, which is elastically deformable when subject to applied pressure. The invention substantially eliminates drift, and other problems, associated with water or other fluid absorption into the tube. The tube may be made of a glass material for encasing a glass fiber. The invention provides low hysteresis. Furthermore, one or more gratings, fiber lasers, or a plurality of fibers may be encased in the coated tube. The grating(s) or reflective elements are xe2x80x9cencasedxe2x80x9d in the tube by having the tube fused to the fiber at the grating area and/or on opposite sides of the grating area adjacent to or at a predetermined distance from the grating. The grating(s) or laser(s) may be fused within the tube, partially within the tube, or to the outer surface of the tube. The invention may be used as an individual (single point) sensor or as a plurality of distributed multiplexed (multi-point) sensors. Also, the invention may be a feed-through design or a non-feed-through design. The tube may have alternative geometries, e.g., a dogbone shape, that provides enhanced force to wavelength shift sensitivity and which is easily scalable for the desired sensitivity.
The invention may be used in harsh environments (i.e., environments having high temperatures and/or pressures), such as in oil and/or gas wells, engines, combustion chambers, etc. In one embodiment, the invention may be an all glass sensor capable of operating at high pressures ( greater than 15 kpsi) and high temperatures ( greater than 150xc2x0 C.). The invention will also work equally well in other applications regardless of the type of environment.
The foregoing and other objects, features, and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof.